1. Field of the Invention
This invention relates to the completion of wells in subterranean formations. In one aspect it relates to an improved method and apparatus for gravel packing the annulus surrounding a wire wrapped liner placed in a well.
2. Description of the Prior Art
A major problem in completing wells in unconsolidated or loosely consolidated formations is sand control. Sand particles entrained in produced fluids can plug flow channels of the formation and can cause severe erosion of well equipment such as lines, the producing string, valves and pumps. A well known sand control technique is gravel packing, whereby properly sized gravel is placed opposite the unconsolidated formation, forming a sand exclusion zone which filters out the sand particles entrained in the produced fluid.
A conventional gravel packing technique involves locating a perforated liner at a subsurface location in the well and thereafter placing gravel around the liner. Normally, a slurry of gravel suspended in a liquid carrier is pumped into the annular space between the formation wall and the liner. As the suspension reaches the bottom of the annulus the gravel is deposited in the annulus on the exterior of the liner and the liquid carrier withdraws through the liner perforations and back up the casing string. In this manner, the gravel builds up until the entire annulus surrounding the liner is filled.
Ideally, the gravel should uniformly and compactly fill the wellbore annulus surrounding the liner. Unfortunately, in some wells, especially deviated wells, the gravel fails to pack uniformly, resulting in voids within the annulus which weaken the pack and permit the production of sand entrained fluids. For example, gravitational forces in deviated wells tend to cause some gravel to prematurely settle out near the upper end of the liner. As a result, a small gravel bank, referred to herein as a dune, begins to form within the upper end of the annulus. As the dune grows and descends down the annulus, more and more of the carrier liquid is diverted through the liner upstream of the dune thereby causing the velocity of the gravel suspension to decline. As velocity drops, the carrier liquid can no longer maintain the gravel in suspension with the result that additional gravel settles out until the dune completely blocks flow to the lower portions of the annulus. Substantially all of the carrier liquid is then diverted into the upstream section of the liner, causing the upper section of the annulus to pack while leaving a substantial void space in the lower section. In practice, a number of gravel dunes and void spaces may be formed in the manner described above.
In order to uniformly compactly fill the annulus surrounding a liner in an inclined wellbore, the upper flow channel must remain open until the lower section of the annulus is filled. One approach to improving gravel packing efficiency is the use of a wide diameter stinger as described in copending application, U.S. Ser. No. 661,662. A stinger is a tube, positioned through the liner, which serves as the return conduit for the carrier liquid. By selecting a wide diameter stinger the annular passage between the interior of the liner and the exterior of the stinger is very restricted. The decreased area available for fluid flow within the annular passage increases the resistance to flow in the passage and decreases the flow of carrier liquid into the liner. If the flow resistance is sufficient to maintain the minimum flow velocity of carrier fluid that is necessary to prevent or stabilize dune formation, then gravel packing efficiency will be greatly enhanced.
One problem with wide diameter stingers is that there may be very little clearance between the stinger and the inside of the liner. For example, clearances can be as little as one-eighth inch, creating the possibility of getting the stinger tightly lodged against the liner or stuck inside the liner. Only a slight dent or bend in the liner can cause this to happen.